In recent years, there is an ever-increasing demand for color image formation apparatuses capable of forming color images. One type of image formation apparatuses capable of forming color images at high speeds has an electrophotographic system of a tandem type. With respect to conventional image formation apparatuses of the tandem type, for example, the inventions disclosed in Japanese Patent Application Laid-Open Nos. 63-81373 and 10-246995 have been known.
Each of the inventions disclosed in the above-mentioned publications, Japanese Patent Application Laid-Open Nos. 63-81373 and 10-246995, has four photosensitive drums each of which has a scanning unit for applying a laser light beam to each of the four photosensitive drums so as to write a latent image thereon.
The four photosensitive drums are placed in parallel with each other in the transporting direction of the paper, which is transported by the transporting belt. Each of these is scanned (main-scanning process) in the direction of the rotary axis by a laser light beam directed from the scanning unit, while being rotated, so that a latent image is written thereon. Here, one line of the latent image written by one main-scanning process is hereinafter referred to as a scanning line.
On the surfaces of the four photosensitive drums bearing the latent images written thereon are supplied toners having respective colors of Y (yellow), M (magenta), C (cyan) and K (black) so as to adhere to the respective latent images. Thus, a toner image having one of the colors is formed on each of the surfaces of the four photosensitive drums. A sheet of paper is successively pressed onto the four photosensitive drums on which toner images have been formed. Consequently, the toner images of the respective colors are successively transferred the sheet of paper to form a color image.
In this case, if there is an offset between the scanning lines constituting the toner images of the respective colors in the color image thus formed, a so-called color offset will occur in the color image, resulting in degradation in the image quality. In order to prevent the color-offset, Japanese Patent Application-Laid-Open No. 10-246955 has proposed an arrangement in which the photosensitive drums are designed to rotate freely, while an annular transport belt is driven to rotate by a motor so that the transport belt is made to contact the photosensitive drums by press-contact rollers installed below the transport belt. The four photosensitive drums are driven to rotate, following the transport belt. At this time, the four photosensitive drums are subjected to the same rotary force so that they are allowed to rotate at the same angular velocity, thereby making it possible to form a color image that is free from positional offsets between the scanning lines.
However, in image formation apparatuses such as printers and copying machines, there will be ever-increasing demands for high resolution (1200 dpi or more).
In contrast, the technique disclosed in Japanese Patent Application Laid-Open No. 10-246995 fails to meet these demands. In other words, in the technique disclosed in Japanese Patent Application Laid-Open No. 10-246995, the image distortion in the sub-scanning direction due to the eccentricity of the rotary axis of the photosensitive drum is corrected by detecting a outer surface dislocation and using the dislocation information continuously. However, when a high resolution is required, the actual amount of correction tends to deviate from the dislocation information in their correlation as the resolution becomes higher.
Moreover, in general, the photosensitive drum tends to have a slight eccentricity due to the limitation in its assembling precision. FIG. 43 is a drawing that shows a state in which a transfer belt is made to contact a photosensitive drum 1801 having such an eccentricity by a contact roller. The photosensitive drum 1801 shown in this Figure has a cross-section that is orthogonal to the rotary axis perpendicular to the paper surface, that passes through point O.
The photosensitive drum 1801 having the eccentricity rotates centered on the center axis passing through point O. In contrast, a transport belt 1802, which has an annular shape, is allowed to move in the direction of arrow A. A contact roller 1803 is made to contact the transport belt 1802 from below while being supported by a spring 1804 so that the transport belt 1802 is made to press-contact the photosensitive drum 1801. A sheet of paper, not shown, is made to press-contact the photosensitive drum 1801 by the press-contact roller 1803 through the transport belt 1802. Thus, a toner image formed on the surface of the photosensitive drum 1801 is transferred onto the sheet of paper.
The distance from point O to the outer surface of the photosensitive drum 1801 having the eccentricity varies depending on the angle of rotation when observed at a fixed point. For this reason, when the center of gravity of the photosensitive drum 1801 is located at G1, the paper and the photosensitive drum 1801 are in contact with each other at press-contact position P1, while when the center of gravity of the photosensitive drum 1801 is located at G2, they are in contact with each other at press-contact position P2.
The press-contact roller 1803 is allowed to move up and down to a certain degree since this is supported by a spring 1804. Since the press-contact roller 1803 also has an eccentricity, the press-contact position varies in a complex manner, thereby giving greater adverse effects to the angular velocity of the photosensitive drum 1801.
When the angular velocity of the photosensitive drum 1801 varies, the distance between scanning lines of a latent image to be written on the photosensitive drum 1801 becomes irregular, resulting in a distortion in the image to be formed. Moreover, in the case of a color image formation apparatus of the tandem type having a plurality of photosensitive drums in which multi-color toner images are superposed so as to form a color image, if the angular velocities of the photosensitive drums deviate, offsets occur in the transferring positions of the toner images of the respective colors, resulting in degradation in the image quality of an image to be formed.
Here, another arrangement has been proposed in which: the transferring position of a toner image is estimated through calculations, and based upon the results thereof, the image forming conditions are adjusted so as to make the transferring positions coincident with each other. However, when the angular velocity of the photosensitive drum 1801 varies, it becomes difficult to accurately estimate the transferring positions of the toner images, resulting in failure to adjust the transferring positions of the toner images through adjustments of the image formation apparatus. Therefore, in the image formation apparatus in which the angular velocity of the photosensitive drum varies, it is impossible to improve the image quality by eliminating the offsets in the transferring positions of the toner images through the adjustments of the image forming conditions.
Moreover, the color image formation apparatus of the tandem type is provided with a writing unit for each of the photosensitive drums. Here, the writing timings and properties of the optical systems in the respective writing units placed in the respective photosensitive drums are not necessarily coincident with each other. For this reason, the writing timing deviates for each of the photosensitive drums, and even when there is no eccentricity in each of the photosensitive drums, there might be deviations in the transferring positions for the toner images of the respective colors.
Furthermore, the radius of each of the photosensitive drums tends to have a slight deviation due to the limitation in the processing precision. In this case also, deviations tend to occur in the transferring positions in the toner images of the respective colors, regardless of the eccentricity of each of the photosensitive drums.
In the future, along with the ever-increasing demands for high-quality printing with high resolution (1200 dpi or more), very high-precision photosensitive drums have to be produced in order to solve the above-mentioned problems. Taking into consideration the development of the technology in the future, it will be possible to improve the processing precision of the photosensitive drum to a certain extent; however, there will be a limitation in the improvement of the processing precision.